This proposal focuses on the functional analysis and characterization of a novel protein that is encoded by PKDH1, the gene responsible for causing autosomal recessive polycystic kidney disease (ARPKD). Several groups have recently identified PKHD1 (Ward et al., 2002; Onuchic et al., 2002; Xiong H, 2002). Of them, two groups independently reported that the longest ORF of PKHD1 contains 67 exons and encodes 4074 amino acids (AAs), and respectively named them fibrocystin and polyductin (FPC), and also demonstrated a large spectrum of alternative variants for PKHD1. By positional cloning, our group has also identified a short form of PKHD1 in which the first 60 exons are nearly identical to PKHD1 (Xiong H, 2002), and we consider it to be an isoform of PKDH1. Using a panel of antibodies, we demonstrated that FPC is widely expressed in epithelial derivatives and is aggregated at basobodies/ciliary subcellular organelles (Zhang et al., 2004). However, the biologic functions of this large and complex protein remain unknown. To elucidate the functional roles of FPC, we will use molecular, cell biological, and transgenic approaches to address the cellular physiology of tubulogenesis and tubular maturation in affected tissues, such as the kidney and liver. We will also address the mechanisms by which an FPC deficiency causes cyst formation and/or fibrosis. Moreover, we have begun to generate mouse models with over-expression of FPC. Together with our other Pkd mutant models, which include null-Pkd1 and -Pkd2 mice, the models will provide the essential tools we need to address several questions. These include whether the absence of Pkhdl gene product(s) causes significant defects in some organs for which the primary duct is essential for their morphogenesis; whether the cystogenesis of ARPKD results from abnormal expression of FPC; whether PKHD1 involves the polycystin pathway; and if so, what the molecular relationship is between FPC and polycystins during embryogenesis, organogenesis, and cystogenesis; whether the functional re-expression of FPC reverse or arrest the cystic phenotypes in FPC-deficient mice. By studying these mice and the cell lines derived from them, we will gain further insight into the role of FPC in the initiation and progression of cyst formation and the regulation of renal epithelial cell physiology. We will also learn more about the relationship between FPC and other PKD-related and -associated genes in the common cystogenic pathway. Ultimately, we hope to use this knowledge to establish a therapeutic basis for this disease.